Ethan Coffel

A Review of Recent Advances in Research on Extreme Heat Events

Reviewing recent literature, we report that changes in extreme heat event characteristics such as magnitude, frequency, and duration are highly sensitive to changes in mean global-scale warming. Numerous studies have detected significant changes in the observed occurrence of extreme heat events, irrespective of how such events are defined. Further, a number of these studies have attributed present-day changes in the risk of individual heat events and the documented global-scale increase in such events to anthropogenic-driven warming. Advances in process-based studies of heat events have focused on the proximate land-atmosphere interactions through soil moisture anomalies, and changes in occurrence of the underlying atmospheric circulation associated with heat events in the midlatitudes. While evidence for a number of hypotheses remains limited, climate change nevertheless points to tail risks of possible changes in heat extremes that could exceed estimates generated from model outputs of mean temperature. We also explore risks associated with compound extreme events and nonlinear impacts associated with extreme heat.

Climate change and the impact of extreme temperatures on aviation

• Air temperature, airport elevation, and aircraft weight influence necessary takeoff speed • Takeoff speed determines necessary runway length • At high temperatures or elevations, aircraft may be weight restricted if runway length is not sufficient.

Projected changes in extreme temperature events based on the NARCCAP model suite

Once-per-year (annual) maximum temperature extremes in North American Regional Climate Change Assessment Program (NARCCAP) models are projected to increase more (less) than mean daily maximum summer temperatures over much of the eastern (western) United States. In contrast, the models almost everywhere project greater warming of once-per-year minimum temperatures as compared to mean daily minimum winter temperatures. Under projected changes associated with extremes of the temperature distribution, Baltimores maximum temperature that was met or exceeded once per year historically is projected to occur 17 times per season by midcentury, a 28% increase relative to projections based on summer mean daily maximum temperature change. Under the same approach, historical once-per-year cold events in Baltimore are projected to occur once per decade. The models are generally able to capture observed geopotential height anomalies associated with temperature extremes in two subregions. Projected changes in extreme temperature events cannot be explained by geopotential height anomalies or lower boundary conditions as reflected by soil moisture anomalies or snow water equivalent.

The Impacts of Rising Temperatures on Aircraft Takeoff Performance

Steadily rising mean and extreme temperatures as a result of climate change will likely impact the air transportation system over the coming decades. As air temperatures rise at constant pressure, air density declines, resulting in less lift generation by an aircraft wing at a given airspeed and potentially imposing a weight restriction on departing aircraft. This study presents a general model to project future weight restrictions across a fleet of aircraft with different takeoff weights operating at a variety of airports. We construct performance models for five common commercial aircraft and 19 major airports around the world and use projections of daily temperatures from the CMIP5 model suite under the RCP 4.5 and RCP 8.5 emissions scenarios to calculate required hourly weight restriction. We find that on average, 10–30% of annual flights departing at the time of daily maximum temperature may require some weight restriction below their maximum takeoff weights, with mean restrictions ranging from 0.5 to 4% of total aircraft payload and fuel capacity by mid- to late century. Both mid-sized and large aircraft are affected, and airports with short runways and high temperatures, or those at high elevations, will see the largest impacts. Our results suggest that weight restriction may impose a non-trivial cost on airlines and impact aviation operations around the world and that adaptation may be required in aircraft design, airline schedules, and/or runway lengths.

Reply to “Comment on ‘Climate Change and the Impact of Extreme Temperatures on Aviation’”

We thank the author of this comment (Hane 2016) for his interest in our work and for his close examination of our study, ‘‘Climate change and the impact of extreme temperatures on aviation’’ (Coffel and Horton 2015). However, we stand by our results in the context of the assumptions and qualifications stated in the original paper, with the core message being that rising temperatures associated with climate change are likely to require increasingly frequent weight restriction of commercial aircraft. The author proposes several technical issues with our study, which we will address in order. First, Hane correctly notes the difference between maximum takeoff weight (MTOW), a hard upper limit on the allowable weight at takeoff, and takeoff weight (TOW), the weight of an aircraft at takeoff on a particular flight. There are many important weight thresholds for commercial aircraft, and in our study we calculate the temperature-induced weight restriction below MTOW (1000, 10 000, or 15 000 lb). Hane’s primary comment, however, is that considering operational takeoff weights by taking into account required fuel and payload would enable more precise estimates of the burden of weight restriction. As Hane also notes, these are not public data and hence were not used. However, in an ongoing follow up study, aircraft performance models are being used to estimate the required fuel for a given flight, allowing a more precise calculation of required weight restriction at a given temperature. Hane notes that a flight from LaGuardia (LGA) to Los Angeles (LAX) would not require a full fuel load for a Boeing 737–800 under normal weather conditions. This is true; however, our goal in this initial study was not to consider specific flights, but rather to illustrate the potential for rising temperatures to impact weight restriction. Second, Hane notes that incremental modifications to the airframe, the engines, or the software can add up to significant increases in performance. This is certainly true, and as we state in our paper, ‘‘changes in technology will no doubt revolutionize the aviation industry in the next 50 years.’’ However, in a warming climate some of these changes may simply help to maintain the same level of takeoff performance as is seen today, rather than to improve it further. Third, Hane notes that engine thrust can in some cases be modestly increased with few or no hardware changes to the aircraft. We agree, and again note that while engines will likely become more efficient over the coming years, the full benefit of these improvements may be tempered by less favorable climate conditions. While we appreciate the details noted in this comment, we emphasize that the goal of our original study was to draw attention to more frequent weight restriction as an issue worth considering by airlines and aircraft manufacturers in their planning efforts. While the details suggested by Hane are important areas for future research, it is important that future studies consider each detail in a comprehensive way. For example, making software enhancements to the engine Corresponding author address: Ethan D. Coffel, 2880 Broadway, New York, NY 10025. E-mail: ec2959@columbia.edu APRIL 2016 CORRES PONDENCE 207

The Science of Adaptation to Extreme Heat

Abstract Heat is a leading cause of weather-related death globally. Recent heat waves have been responsible for tens of thousands of excess deaths, damage to infrastructure, crop losses, and economic disruption. Many of these events have been in part attributed to climate change and in coming decades the frequency, magnitude, and duration of extreme heat waves are very likely to rise across the world. Given the high probability of increasingly severe temperatures and their moderate to high impact on human health, heat impacts are projected to expand dramatically. This chapter examines the state of the science concerning heat stress and its impacts on human health, the urban heat island, and heat adaptation strategies in both low- and high-income countries, and considers how adaptation can be guided by evidence to yield improved results. Adaptation solutions based on sound science—physical, social, and behavioral—are essential to target the most vulnerable populations and decrease impacts on human health and well-being.